Structural changes to support atomic installs

This change makes the initial changes to installPackage that allow
verification of multiple packages for install and a single commit of all
of them atomically. Install will eventually be broken up into 3 stages:

  * scan: only parse and verify the APK; isolated from system state
  * reconcile: verify that the APK is compatible with the system given
               current state and with the other apps being installed
	       along side it
  * commit: modify system state to account for the reconciled apps

Test: manual install still works; non-functional change
Bug: 109941548
Change-Id: I84cc2185942879d7d0217d49973404bb061918af